ip route: enable per-route ecn settings via 'features' option

This permits to selectively enable explicit congestion notification via
the routing table.

If this ecn feature is not set, the kernel will use the tcp_ecn sysctl
to decide wheter to use ECN when establising a TCP connection.

At the time of this writing, the kernel supports ecn and allfrags, but
allfrags is of dubious value and not implemented here.

Example:

ip route change 192.168.2.0/24 dev eth0 features ecn
Signed-off-by: Florian Westphal <fw@strlen.de>

ip/iproute.c

@@ -80,6 +80,7 @@ static void usage(void)
 	fprintf(stderr, "           [ window NUMBER] [ cwnd NUMBER ] [ initcwnd NUMBER ]\n");
 	fprintf(stderr, "           [ ssthresh NUMBER ] [ realms REALM ] [ src ADDRESS ]\n");
 	fprintf(stderr, "           [ rto_min TIME ] [ hoplimit NUMBER ] [ initrwnd NUMBER ]\n");
+	fprintf(stderr, "           [ features FEATURES ]\n");
 	fprintf(stderr, "           [ quickack BOOL ]\n");
 	fprintf(stderr, "TYPE := [ unicast | local | broadcast | multicast | throw |\n");
 	fprintf(stderr, "          unreachable | prohibit | blackhole | nat ]\n");
@@ -89,6 +90,7 @@ static void usage(void)
 	fprintf(stderr, "RTPROTO := [ kernel | boot | static | NUMBER ]\n");
 	fprintf(stderr, "TIME := NUMBER[s|ms]\n");
 	fprintf(stderr, "BOOL := [1|0]\n");
+	fprintf(stderr, "FEATURES := ecn\n");
 	exit(-1);
 }
 
@@ -280,6 +282,19 @@ static int calc_host_len(const struct rtmsg *r)
 		return -1;
 }
 
+static void print_rtax_features(FILE *fp, unsigned int features)
+{
+	unsigned int of = features;
+
+	if (features & RTAX_FEATURE_ECN) {
+		fprintf(fp, " ecn");
+		features &= ~RTAX_FEATURE_ECN;
+	}
+
+	if (features)
+		fprintf(fp, " 0x%x", of);
+}
+
 int print_route(const struct sockaddr_nl *who, struct nlmsghdr *n, void *arg)
 {
 	FILE *fp = (FILE*)arg;
@@ -535,6 +550,9 @@ int print_route(const struct sockaddr_nl *who, struct nlmsghdr *n, void *arg)
 
 			val = *(unsigned*)RTA_DATA(mxrta[i]);
 			switch (i) {
+			case RTAX_FEATURES:
+				print_rtax_features(fp, val);
+				break;
 			case RTAX_HOPLIMIT:
 				if ((int)val == -1)
 					val = 0;
@@ -885,6 +903,20 @@ static int iproute_modify(int cmd, unsigned flags, int argc, char **argv)
 			if (get_unsigned(&win, *argv, 0))
 				invarg("\"initrwnd\" value is invalid\n", *argv);
 			rta_addattr32(mxrta, sizeof(mxbuf), RTAX_INITRWND, win);
+		} else if (matches(*argv, "features") == 0) {
+			unsigned int features = 0;
+
+			while (argc > 0) {
+				NEXT_ARG();
+
+				if (strcmp(*argv, "ecn") == 0)
+					features |= RTAX_FEATURE_ECN;
+				else
+					invarg("\"features\" value not valid\n", *argv);
+				break;
+			}
+
+			rta_addattr32(mxrta, sizeof(mxbuf), RTAX_FEATURES, features);
 		} else if (matches(*argv, "quickack") == 0) {
 			unsigned quickack;
 			NEXT_ARG();

man/man8/ip-route.8.in

@@ -113,6 +113,8 @@ replace " } "
 .IR NUMBER " ] [ "
 .B  initrwnd
 .IR NUMBER " ] [ "
+.B  features
+.IR FEATURES " ] [ "
 .B  quickack
 .IR BOOL " ]"
 
@@ -140,6 +142,10 @@ throw " | " unreachable " | " prohibit " | " blackhole " | " nat " ]"
 .BR kernel " | " boot " | " static " |"
 .IR NUMBER " ]"
 
+.ti -8
+.IR FEATURES " := [ "
+.BR ecn " | ]"
+
 
 .SH DESCRIPTION
 .B ip route
@@ -410,6 +416,18 @@ the initial receive window size for connections to this destination.
 Actual window size is this value multiplied by the MSS of the connection.
 The default value is zero, meaning to use Slow Start value.
 
+.TP
+.BI features " FEATURES " (3.18+ only)
+Enable or disable per-route features.  Only available feature at this
+time is
+.B ecn
+to enable explicit congestion notification when initiating connections to the
+given destination network.
+When responding to a connection request from the given network, ecn will
+also be used even if the
+.B net.ipv4.tcp_ecn
+sysctl is set to 0.
+
 .TP
 .BI quickack " BOOL " "(3.11+ only)"
 Enable or disable quick ack for connections to this destination.